>>>It maybe be your principle account, if you switch main account to another account, you maybe able to delete that trouble one.
>>
>>Unfortunately I am unable to remove my old account. When I try to do so, I get this message (see attached). I don't know how to remove all old Exchange accounts first.
>
>The message seems to indicate that it is still your primary account.

I'm not sure what am I supposed to do to get rid of it. In the Outlook account settings my current account is set correctly as primary (see attached). Yet when I try to remove the other one I get this message.

BTW, I went ahead and cleared all old appointments in my old calendar.